Generally, a capacitive microphone utilizes a capacitance between a pair of electrodes facing each other to transmit an acoustic signal. The capacitive microphone can be manufactured by a semiconductor MEMS process to have an ultra-small size.
A MEMS microphone includes a bendable diaphragm and a back plate facing the diaphragm. The diaphragm is disposed apart from a substrate and the back plate to freely bend upward or downward in response to sound waves incident upon the diaphragm. The diaphragm can be a membrane structure to sense an acoustic pressure to create a displacement. In other words, when the acoustic pressure arrives at the diaphragm, the diaphragm may be bent toward the back plate due to the acoustic pressure. The displacement of the diaphragm can be sensed through a change of capacitance formed between the diaphragm and the back plate. As a result, sound can be converted into an electrical signal for output.
A device characteristic in the MEMS microphone can be identified through a value of capacitance using a C-V measurement. In other words, when a voltage is applied to the diaphragm, the diaphragm having a semi-spherical shape can be bent toward the back plate to generate a capacitance. Contrariwise, when the voltage being applied to the diaphragm is off, the diaphragm can return to an initial position such that the value of capacitance between the diaphragm and the back plate becomes zero.
However, even though the applied voltage is off, the diaphragm cannot return to the initial position to stay bent. Thus, a buckling phenomenon may occur such that there is no change in the value of the capacitance.
The buckling phenomenon may occur due to a stress of the diaphragm. That is, the diaphragm can have an ideal stress in case when the diaphragm is kept horizontally or is bent by under 0.5 μm. However, when the diaphragm is bent by over 0.5 μm, the diaphragm may have a compressive stress to cause the buckling phenomenon occur such that the MEMS microphone may malfunction.